Using nuclear techniques to investigate volumes of either organic or inorganic matter is well known. For example, naturally radioactive elements are commonly found in earth formations in various proportions, depending on the type of lithology and other factors. Radioactive isotopes of potassium, uranium, and thorium are typical regularly occurring naturally radioactive elements commonly quantified in the petroleum industry.
A rigid or non-rigid conveyance device is often used to convey one or more nuclear radiation detectors into a borehole intersecting the earth formation, often as part of a tool or a set of tools. There, the detector(s) may be used to estimate radiation associated with a volume of interest of the earth formation by generating a response to the one or more types of nuclear radiation detected. The response may be indicative of the nuclear radiation (e.g., gamma rays) present in the near-borehole environment during measurement. This response may then be used to estimate a parameter of interest of the volume. In nuclear logging, additional radioactive isotopes may be generated by providing radiation (e.g., neutron bombardment) to the formation. The detected nuclear radiation may be expressed as an energy spectrum (the “response spectrum”). “Response spectrum” refers to not only the response spectrum as originally acquired, but also after contrast enhancement, corrections, or pre-processing is applied.
Oil well logging has been known for many years and provides an oil and gas well driller with information about the particular earth formation being drilled. One or more parameters of interest of the earth formation obtained as above may relate to hydrocarbon exploration or production. For example, the parameter of interest may provide indications of the location and extent of hydrocarbons in a given earth formation.